低围压循环荷载作用下饱和粗粒土的动力特性与骨干曲线模型研究

Dynamic behavior and backbone curve model of saturated coarse-grained soil under cyclic loading and low confining pressure

基床层是铁路路基的核心组成部分,一般为粗颗粒土,厚约2.5~3.0 m,长期直接承受行车荷载的反复作用,其在动载反复作用下的变形特性是评价路基工作性能的关键要素之一。为研究粗粒土在列车循环荷载作用下的应力-应变特性,开展了一系列应力控制的单向循环加载大型动三轴试验,模拟列车动载和路基粗粒料填筑实际情况,包括不同动应力幅值（模拟不同列车轴重）、不同围压（模拟不同埋深）的动三轴持续振动试验。结果表明,在循环荷载作用下,土体刚度变化与振动次数、围压关系密切。根据动应力幅值大小的不同,循环荷载作用下饱和粗粒土的动应变随振次的发展形态可分为3种类型：稳定型、破坏型和临界型。根据试验所得出的动应力-应变关系曲线特点,建立了含围压和循环振次的骨干曲线模型。与传统的骨干曲线模型相比,该模型能反映土体刚度随循环振次的变化,更能反映列车往复作用的实际情况;同时该模型能用于估算路基土体动强度,对铁路路基核心层的动力变形稳定性评价和基于动力变形控制的路基设计具有参考价值。

The subgrade bed layer is the core part of railway subgrade composed of coarse-grained soil（CGS）, which is generally about 2.5-3.0 m thick. It is this layer that is directly subjected to long-term effect of repeated traffic loading. The deformation of the subgrade bed layer under cyclic dynamic loading is one of the key factors to evaluate the operating performance of subgrade. To study the stress-strain characteristics of CGS under cyclic loading, a series of large-scale dynamic triaxial tests is carried out at different confining pressures（simulating different depths under the effect of lateral pressure） and dynamic stress amplitudes（simulating different axle loads）, to simulate dynamic loading of train and actual situation of CGS filling by one-way cyclic stress-controlled loading. The results show that, under the cyclic loading, the change for soil stiffness is closely related to the vibration number and confining pressure. According to different dynamic stress amplitudes, the dynamic strain of saturated CGS under cyclic loading can be divided into three types according to development trends of vibration times： stable, destructive and critical. Based on the experimental results, a backbone curve model is developed including confining pressure and vibration times. Compared with the traditional backbone curve model, the proposed model can reflect the variation of soil stiffness with vibration times and the actual situation of cyclic effect of train. In addition, the model can also be used to determine the dynamic strength of subgrade soil, providing reference for evaluating dynamic stress deformation stability for subgrade bed core layer of railway and subgrade bed design based on dynamic deformation control.